gnu-smalltalk: add test

Closes Homebrew/legacy-homebrew#26502.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Formula/gnu-smalltalk.rb

@@ -60,4 +60,13 @@ def install
     system 'make', '-j1', 'check' if build.include? 'tests'
     system "make install"
   end
+
+  test do
+    path = testpath/"test.gst"
+    path.write "0 to: 9 do: [ :n | n display ]\n"
+
+    output = `#{bin}/gst #{path}`.strip
+    assert_equal "0123456789", output
+    assert_equal 0, $?.exitstatus
+  end
 end